Instead of directly calling into the back-to-bootrom code, the RK3399
common SPL implementation now uses BOOT_DEVICE_BOOTROM to trigger a
transfer back into the bootrom.

With this factored out, the spl_board_init function can not be
customised for each RK3399 board.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>